This invention relates to power converters, and more particularly to high efficiency high voltage dc or ac input to low voltage dc output power converters.
Maximizing efficiency is very import in power converters. Most power systems are less efficient than they could be. Important contributors to inefficiency are: too many conversion steps, and operating at duty-cycles that have high rms current and/or voltage. Often, the latter is necessitated by specifying operation over too wide a range of input voltage.
Power converter can be optimized for the nominal operating conditions. The most efficient topology is used, provided that it must have acceptable performance during transient conditions.
Many power converters operate with a well-defined normal input voltage, either from commercial ac power or from a regulated up-stream power converter. Efficiency should be optimized for that normal voltage.
As the input voltage deviates further from normal, more compromise is acceptable. Three conditions are generally applicable, as shown in FIG. 2. Special applications may have more.                1. Normal steady-state voltage: This is a narrow band around the nominal value, probably 3 to 5 percent. Commercial ac power is quite well regulated, as are many 48 V dc systems. Efficiency is maximized for this condition.        2. Normal transient voltage: This is a much wider voltage band, and it may vary with time. The power converter must meet all performance specifications, but operation in this condition is limited in time, and it is acceptable to have degraded efficiency.        3. Abnormal transient: Operation need not meet specification, but it is preferred that 1) the power converter will recover following the transient with no damage, and 2) there will be no over-voltage output.        
Special applications may require emergency modes, such as prolonged operation at reduced voltage (brown-out) or on battery backup. Many applications will require continued operation for a time following loss of power (hold-up).
A power converter is more efficient if it is designed for one input voltage. If connected to an incorrect voltage, it is preferred that it not be damaged and there is no over-voltage output, but it is probably acceptable if the power converter remains off (disabled). Some power supplies are required to work with “universal” input, usually 100 V to 240 V ac. An auto-ranting input transformer can be designed for dual or triple input voltages, allowing auto-ranging between two voltages, for example, 100V ac, 120 V ac or 240 V ac, with no compromise in efficiency, as long as it is operated near the nominal voltage for each range.